Compressor units of the above mentioned type are used in a variety of applications in wide ranges of sizes. Of special interest is the application of an electrically driven compressor in combination with a gas tight or hermetically closed casing enclosing the motor and the compressor, so that the necessity for the use of a rotor sealing is avoided. Since a rotor sealing is always complicated and expensive especially due to the normally connected auxiliary system, which is usually needed to provide a sealing fluid, the sealless application is of an enormous interest. Further reasons for the effort to obtain a sealless machine are new fields of application for compressor units. In general the new applications concentrate on the avoidance of media exchange between a process fluid to be compressed and the surrounding. One possible usage is the compression of toxic fluids, for which the auxiliary system to provide a sealing fluid for shaft sealing is extremely complicated. Another even more interesting application is the compression of gas, in particular natural gas, below sea level respectively with surrounding sea water. The economic interest in these compressor units is increasingly high due to the shortage of fossil fuels and the new possibility to exploit natural gas resources in the ocean more efficiently.
However, the new development of compressing natural gas below sea level directly at the well head goes along with enormous technical problems connected with difficult operating conditions and bad accessibility of the compressor units. Major challenges are low temperatures near the freezing point, high pressure of the surrounding chemically aggressive sea water, non purified and chemically aggressive fluids to be compressed, wherein the natural gas varies in the suction pressure and carries next to foreign particles also non gaseous fluids respectively liquids in significant amounts.
Further, over the life time of the well, the pressure of the natural gas to be exploited decreases significantly and due to environmental protection no media exchange between the compressor unit and the surrounding is admitted.